Method and apparatus of multi-energy imaging

ABSTRACT

The present invention is directed to a method and apparatus of multi-energy data acquisition. An imaging system is also provided and includes a number of HF electromagnetic energy filters. The filters include at least a first and a second filter wherein the first filter is positioned in a path of HF electromagnetic energy when an HF electromagnetic energy source is energized to a first voltage and the second filter is positioned in the path of HF electromagnetic energy when the HF electromagnetic energy source is energized to a second voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority of U.S.Ser. No. 10/063,366 filed Apr. 16, 2002, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to diagnostic imaging and, moreparticularly, to a method and apparatus of acquiring imaging data atmore than one energy range using multi-energy high speed switchingfilters.

Typically, in computed tomography (CT) imaging systems, an x-ray sourceemits a fan-shaped beam toward a subject or object, such as a patient ora piece of luggage. Hereinafter, the terms “subject” and “object” shallinclude anything capable of being imaged. The beam, after beingattenuated by the subject, impinges upon an array of radiationdetectors. The intensity of the attenuated beam radiation received atthe detector array is typically dependent upon the attenuation of thex-ray beam by the subject. Each detector element of the detector arrayproduces a separate electrical signal indicative of the attenuated beamreceived by each detector element. The electrical signals aretransmitted to a data processing system for analysis which ultimatelyproduces an image.

Generally, the x-ray source and the detector array are rotated about thegantry within an imaging plane and around the subject. X-ray sourcestypically include x-ray tubes, which emit the x-ray beam at a focalpoint. X-ray detectors typically include a collimator for collimatingx-ray beams received at the detector, a scintillator for convertingx-rays to light energy adjacent the collimator, and photodiodes forreceiving the light energy from the adjacent scintillator and producingelectrical signals therefrom.

Typically, each scintillator of a scintillator array converts x-rays tolight energy. Each scintillator discharges light energy to a photodiodeadjacent thereto. Each photodiode detects the light energy and generatesa corresponding electrical signal. The outputs of the photodiodes arethen transmitted to the data processing system for image reconstruction.

Recently, dual energy CT scanning commonly referred to as“tomochemistry” has increasingly been used as a means of gainingdiagnostic information of a subject. A principle objective of dualenergy scanning is to obtain diagnostic CT images that enhance contrastseparation within the image by utilizing two scans at differentchromatic energy states. A number of techniques have been proposed toachieve dual energy scanning including a “Two Crystal” method and a “TwokV” method. These two techniques were discussed by F. Kelcz, et al. inan article in Medical Physics 6(5), SEP./OCT. (1979) entitled “NoiseConsiderations in Dual Energy CT Scanning.” With respect to the “Two kV”technique, high frequency generators have made it possible to switch thekVp potential of the high frequency electromagnetic energy projectionsource on alternating views. As a result, data for two dual energyimages may be obtained in a temporarily interleaved fashion rather thantwo separate scans made several seconds apart as required with previousCT technology. Simply scanning at two kVp potentials in an interleavedmanner is not desirable as filtration of the dual energy levels remainsa concern. For example, dual energy CT scanning with fixed filtrationresults in a dramatic decrease in signal strength when comparing the 80kVp spectrum to the 140 kVp spectrum. Furthermore, the effective energyseparation between the two spectrums is approximately 25 kV. Selectivelyfiltering each kVp spectrum with different x-ray filtration can increasethe energy separation to 45 kV in this case. This dramatically improvesthe effectiveness of dual energy CT imaging.

Therefore, it would be desirable to design an apparatus and method foracquiring imaging data at more than one energy state during a singlescan without jeopardizing signal strength.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a method and apparatus ofmulti-energy imaging overcoming the aforementioned drawbacks. A set ofrotatable filters is provided and controlled by a controller configuredto position each filter in a path of high frequency electromagneticenergy synchronously with a changing kVp cycle. By using a pulsed highfrequency electromagnetic energy source together with the set offilters, a burst of high frequency electromagnetic energy beams may begenerated at a desired energy/filtration combination for each view. Aset of views properly filtered for the high frequency electromagneticenergy implemented may then be generated and used for imagereconstruction.

Therefore, in accordance with one aspect of the present invention, a CTsystem comprises a rotatable gantry having an opening for receiving asubject to be scanned. The CT system further includes a high frequencyelectromagnetic energy source configured to project a number of highfrequency electromagnetic energy beams towards the subject. A generatoris also provided and configured to energize the high frequencyelectromagnetic energy source to at least a first energy state and asecond energy state. The CT system also includes a number of highfrequency electromagnetic energy filters positionable between the highfrequency electromagnetic energy source and the subject. The number ofhigh frequency electromagnetic energy filters includes at least a firstfilter and a second filter wherein the first filter is positionedbetween the high frequency electromagnetic energy source and the subjectwhen the high frequency electromagnetic energy source is energized tothe first energy state. The second filter is configured to be positionedbetween the high frequency electromagnetic energy source and the subjectwhen the high frequency electromagnetic energy source is energized tothe second energy state.

In accordance with a further aspect of the present invention, acontroller is configured to acquire CT imaging data at more than onechromatic energy state. The controller has instructions to energize ahigh frequency electromagnetic energy source configured to project ahigh frequency electromagnetic energy beam toward a subject to bescanned to a first voltage potential. The controller has furtherinstructions to position a first portion of a filtering apparatusbetween the subject and the high frequency electromagnetic energy sourcealong a path of rotation during energization of the high frequencyelectromagnetic energy source to the first voltage potential. Thecontroller also has instructions to energize the high frequencyelectromagnetic energy source to a second voltage potential. Thecontroller is then instructed to position a second portion of thefiltering apparatus between the subject and the high frequencyelectromagnetic energy source along the path of rotation duringenergization of the high frequency electromagnetic energy source to thesecond voltage potential.

In accordance with a further aspect of the present invention, a methodof acquiring imaging data at more than one chromatic energy comprisesthe step of projecting a first beam of electromagnetic energy along aprojection path toward a subject. The method further includes the stepof positioning a first filter in the projection path during projectionof the first beam. The method also includes projecting a second beam ofelectromagnetic energy along the projection path toward the subject andpositioning a second filter in the projection path during projection ofthe second beam of electromagnetic energy.

In accordance with yet a further aspect of the present invention, acomputer readable storage medium has a computer program stored thereon.The computer program represents a set of instructions that when executedby a computer causes the computer to energize the high frequencyelectromagnetic energy source to a first voltage to cause the highfrequency electromagnetic energy source to project a first beam ofelectromagnetic energy toward the subject to be scanned. The computer isfurther caused to position a first filter between the high frequencyelectromagnetic energy source and the subject during energization of thehigh frequency electromagnetic energy source to the first voltage. Theset of instructions further causes the computer to energize the highfrequency electromagnetic energy source to a second voltage to cause thehigh frequency electromagnetic energy source to project a second beam ofelectromagnetic energy toward the subject and position a second filterbetween the electromagnetic energy source and the subject duringenergization of the high frequency electromagnetic energy source to thesecond voltage.

In accordance with yet another aspect of the present invention, afiltering apparatus for a radiation emitting imaging system is provided.The filtering apparatus includes a hub having a number of connectionports and a first filter connected to the hub at a first connection portand a second filter connected to the hub at a second connection port.The first filter is configured to have a first filtering power and thesecond filter is configured to have a second filtering power.

Various other features, objects and advantages of the present inventionwill be made apparent from the following detailed description and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a pictorial view of a CT imaging system.

FIG. 2 is a block schematic diagram of the system illustrated in FIG. 1.

FIG. 3 is a perspective view of one embodiment of a CT system detectorarray.

FIG. 4 is a perspective view of one embodiment of a detector.

FIG. 5 is illustrative of various configurations of the detector in FIG.4 in a four-slice mode.

FIG. 6 is a perspective view of one embodiment of a filtering apparatusin accordance with the present invention.

FIG. 7 illustrates positioning of each filter of the filtering apparatusas a function of energy applied to a high frequency electromagneticenergy projection source.

FIG. 8 is a cross-sectional view of an alternate embodiment of afiltering apparatus in accordance with the present invention.

FIG. 9 is a pictorial view of a CT system for use with a non-invasivepackage inspection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The operating environment of the present invention is described withrespect to a four-slice computed tomography (CT) system. However, itwill be appreciated by those skilled in the art that the presentinvention is equally applicable for use with single-slice or othermulti-slice configurations. Moreover, the present invention will bedescribed with respect to the detection and conversion of x-rays.However, one skilled in the art will further appreciate, that thepresent invention is equally applicable for the detection and conversionof other high frequency electromagnetic energy. The present inventionwill be described with respect to a “third generation” CT scanner, butis equivalently applicable with other CT systems.

Referring to FIGS. 1 and 2, a computed tomography (CT) imaging system 10is shown as including a gantry 12 representative of a “third generation”CT scanner. Gantry 12 has an x-ray source 14 that projects a beam ofx-rays 16 toward a filtering apparatus 17 and a detector array 18 on theopposite side of the gantry 12. The filtering apparatus 17 may include apre-patient filter, a post-patient filter, or both. In FIGS. 1 and 2,the filtering apparatus 17 is shown as a pre-patient filter, as will bedescribed more fully with respect to FIGS. 6-8. Detector array 18 isformed by a plurality of detectors 20 which together sense the projectedx-rays that pass through a medical patient 22. Each detector 20 producesan electrical signal that represents the intensity of an impinging x-raybeam and hence the attenuated beam as it passes through the patient 22.During a scan to acquire x-ray projection data, gantry 12 and thecomponents mounted thereon including source 14, filtering apparatus 17,and detector array 18 rotate about a center of rotation 24.

Rotation of gantry 12 and the operation of x-ray source 14 are governedby a control mechanism 26 of CT system 10. As will be described below,control mechanism 26 includes a filter controller 27 that providespositioning signals to filtering apparatus 17. Control mechanism 26 alsoincludes an x-ray controller 28 that provides power and timing signalsto an x-ray source 14 and a gantry motor controller 30 that controls therotational speed and position of gantry 12. A data acquisition system(DAS) 32 in control mechanism 26 samples analog data from detectors 20and converts the data to digital signals for subsequent processing. Animage reconstructor 34 receives sampled and digitized x-ray data fromDAS 32 and performs high speed reconstruction. The reconstructed imageis applied as an input to a computer 36 which stores the image in a massstorage device 38.

Computer 36 also receives commands and scanning parameters from anoperator via console 40 that has a keyboard or other data entry module.An associated display 42 allows the operator to observe thereconstructed image and other data from computer 36. The operatorsupplied commands and parameters are used by computer 36 to providecontrol signals and information to DAS 32, x-ray controller 28 andgantry motor controller 30. In addition, computer 36 operates a tablemotor controller 44 which controls a motorized table 46 to positionpatient 22 and gantry 12. Particularly, table 46 moves portions ofpatient 22 through a gantry opening 48.

As shown in FIGS. 3 and 4, detector array 18 includes a plurality ofphotodiodes 60 forming a photodiode array 52 and a plurality ofscintillators 57 forming a scintillator array 56. A collimator (notshown) is positioned above scintillator array 56 to collimate x-raybeams 16 before such beams impinge upon scintillator array 56.

In one embodiment, shown in FIG. 3, detector array 18 includes 57detectors 20, each detector 20 having an array size of 16×16. As aresult, array 18 has 16 rows and 912 columns (16×57 detectors) whichallows 16 simultaneous slices of data to be collected with each rotationof gantry 12.

Switch arrays 80 and 82, FIG. 4, are multi-dimensional semiconductorarrays coupled between scintillator array 56 and DAS 32. Switch arrays80 and 82 include a plurality of field effect transistors (FET) (notshown) arranged as multi-dimensional array. The FET array includes anumber of electrical leads connected to each of the respectivephotodiodes 60 and a number of output leads electrically connected toDAS 32 via a flexible electrical interface 84. Particularly, aboutone-half of photodiode outputs are electrically connected to switch 80with the other one-half of photodiode outputs electrically connected toswitch 82. Additionally, a reflector material 59 is interstitiallydisposed between each scintillator 57 to reduce light spreading fromadjacent scintillators. Each detector 20 is secured to a detector frame77, FIG. 3, by mounting brackets 79.

Switch arrays 80 and 82 further include a decoder (not shown) thatenables, disables, or combines photodiode outputs in accordance with adesired number of slices and slice resolutions for each slice. Decoder,in one embodiment, is a decoder chip or a FET controller as known in theart. Decoder includes a plurality of output and control lines coupled toswitch arrays 80 and 82 and DAS 32. In one embodiment defined as a 16slice mode, decoder enables switch arrays 80 and 82 so that all rows ofthe photodiode array 52 are activated, resulting in 16 simultaneousslices of data for processing by DAS 32. Of course, many other slicecombinations are possible. For example, decoder may also select fromother slice modes, including one, two, and four-slice modes.

As shown in FIG. 5, by transmitting the appropriate decoderinstructions, switch arrays 80 and 82 can be configured in thefour-slice mode so that the data is collected from four slices of one ormore rows of photodiode array 52. Depending upon the specificconfiguration of switch arrays 80 and 82, various combinations ofphotodiodes 60 can be enabled, disabled, or combined so that the slicethickness may consist of one, two, three, or four rows of scintillatorarray elements 57. Additional examples include, a single slice modeincluding one slice with slices ranging from 1.25 mm thick to 20 mmthick, and a two slice mode including two slices with slices rangingfrom 1.25 mm thick to 10 mm thick. Additional modes beyond thosedescribed are contemplated.

Referring now to FIG. 6, a four-spoked filtering apparatus 17 is shown.While a four-spoked filtering apparatus will be described, the presentinvention is not so limiting and therefore a filtering apparatus withless than four filter spokes as well as a filtering apparatus with morethan four filter spokes are contemplated and within the scope of thepresent invention. Filtering apparatus 17 includes a hub 86 having anumber of connection ports 88. In this embodiment, a connection port 88is positioned every 90° around the hub. Hub 86 is shown as a cylinderbut a spherical hub is also contemplated. In a three-spoked filteringapparatus, each connection port would be located every 120° along thehub. Connected to hub 86 at each connection port 88 is a filter 90. Eachfilter member 90 may be connected to hub 86 at connection port 88 in anumber of known manners. For example, filtering member 92-96 may besnap-fit, bolted, or integrated with hub 86 as a single integral body.In this embodiment, each connection port 86 includes a slot forreceiving a filter but other receiving designs are contemplated. As willbe described below, each filtering member 92-96 has a differingfiltering power. That is, filtering member 90 has a filtering powerdifferent from the filtering power of filter 94 and so forth.

Hub 86 includes circuitry (not shown) that responds to electricalsignals generated by filter controller 27, FIG. 2, to position one ofthe filters 90-96 in a path of high frequency electromagnetic energy.Hub 86 is thereby caused to rotate filters 90-96 into the high frequencyelectromagnetic energy path synchronously with energization of the highfrequency electromagnetic energy projection source 14 of FIG. 1. Thesynchronous relationship of filter position and source energization isbest shown in FIG. 7.

Shown in FIG. 7 are three plots illustrating the position of each filterwith respect to the kVp potential of the high frequency electromagneticenergy source. As shown, a first filter, such as filter 90, ispositioned in the path of high frequency electromagnetic energy when thehigh frequency electromagnetic energy source projects a burst of highfrequency electromagnetic energy having a kVp potential A. The x-raysource then emits a second burst of x-rays having a kVp potential B andsimultaneously therewith filter 92 is positioned in the x-ray path.Thereafter, the x-ray source emits another burst of x-rays having a kVppotential C. Simultaneously therewith, filter 94 is rotated by the hubinto the x-ray path. The x-ray source is then instructed to emit anotherburst of x-rays having a kVp potential D. When the x-ray sourcegenerates the burst of x-rays at potential D, the filter controllertransmits a signal to the filtering apparatus to position filter 96 inthe x-ray path.

FIG. 7 illustrates the positioning of each filter in the x-ray pathdepending upon the kVp potential of the beam of x-rays. However, thepresent invention does not require the sequential placement of eachfilter as described above. That is, depending upon the requirements ofthe imaging protocol filter 96 may be positioned in the x-ray path afterfilter 90. Ultimately, the order by which the filters are positioned inthe x-ray path is not limited to the sequential description providedabove.

Referring now to FIG. 8, a filtering apparatus 98 in accordance withanother embodiment of the present invention is shown. Filteringapparatus 98 may be used with or independently of filtering apparatus 17specifically described in FIG. 6. In the illustrated embodiment,filtering apparatus 98 includes a single filter comprising a number offiltering sections 100-106. Each filtering section 100-106 has adifferent filtering power. Therefore, filtering apparatus 98 may be usedfor multi-energy CT scanning. Filtering apparatus 98 is designed suchthat one section 100-106 is positioned in the x-ray path depending uponthe kVp potential of the x-ray beam. For example, when the x-ray beamhas a high kVp potential filtering section 100 may be positioned in thex-ray path. Whereas, when the x-ray beam has a lower kVp potentialfiltering section 106 is positioned within the x-ray path. Filteringapparatus 98 is shown as comprising four separate and distinct filteringsections. However, the present invention is not limited to only foursections and therefore a filtering apparatus with less than foursections or more than four sections is equivalently applicable with thepresent invention.

In an alternate embodiment of the present invention, the filteringapparatus 98 shown in FIG. 8 is incorporated with the four-spokedfiltering apparatus 17 illustrated in FIG. 6. With this embodiment, eachfilter 90-96 has a number of filtering sections similar to that shown inFIG. 8. As a result, the number of filtering combinations can beincreased without requiring multiple filtering apparatuses. For example,in a four-spoked filtering apparatus wherein each filter has fourfiltering sections, a total of sixteen filtering combinations may beutilized in acquiring imaging data.

Referring now to FIG. 9, package/baggage inspection system 100 includesa rotatable gantry 102 having an opening 104 therein through whichpackages or pieces of baggage may pass. The rotatable gantry 102 housesa high frequency electromagnetic energy source 106 and a filteringapparatus 107, as well as a detector assembly 108. Filtering apparatus107 is shown as being positioned between the source 106 and object 116.However, filtering apparatus 107 could be placed between object 116 anddetector assembly 108. In another embodiment, a first filter ispositioned pre-object and a second filter positioned post-object. Aconveyor system 110 is also provided and includes a conveyor belt 112supported by structure 114 to automatically and continuously passpackages or baggage pieces 116 through opening 104 to be scanned.Objects 116 are fed through opening 104 by conveyor belt 112, imagingdata is then acquired, and the conveyor belt 112 removes the packages116 from opening 104 in a controlled and continuous manner. As a result,postal inspectors, baggage handlers, and other security personnel maynon-invasively inspect the contents of packages 116 for explosives,knives, guns, contraband, etc. by viewing the images on a display (notshown).

Therefore, in accordance with one embodiment of the present invention, aCT system comprises a rotatable gantry having an opening for receiving asubject to be scanned. The CT system further includes a high frequencyelectromagnetic energy source configured to project a number of highfrequency electromagnetic energy beams towards the subject. A generatoris also provided and configured to energize the high frequencyelectromagnetic energy source to at least a first energy state and asecond energy state. The CT system also includes a number of highfrequency electromagnetic energy filters positionable between the highfrequency electromagnetic energy source and the subject. The number ofhigh frequency electromagnetic energy filters includes at least a firstfilter and a second filter wherein the first filter is positionedbetween the high frequency electromagnetic energy source and the subjectwhen the high frequency electromagnetic energy source is energized tothe first energy state. The second filter is configured to be positionedbetween the high frequency electromagnetic energy source and the subjectwhen the high frequency electromagnetic energy source is energized tothe second energy state.

In accordance with a further embodiment of the present invention, acontroller is configured to acquire CT imaging data in more than onechromatic energy state. The controller has instructions to energize ahigh frequency electromagnetic energy source configured to project ahigh frequency electromagnetic energy beam toward a subject to bescanned to a first voltage potential. The controller has furtherinstructions to position a first portion of a filtering apparatusbetween the subject and the high frequency electromagnetic energy sourcealong a path of rotation during energization of the high frequencyelectromagnetic energy source to the first voltage potential. Thecontroller also has instructions to energize the high frequencyelectromagnetic energy source to a second voltage potential. Thecontroller is then instructed to position a second portion of thefiltering apparatus between the subject and the high frequencyelectromagnetic energy source along the path of rotation duringenergization of the high frequency electromagnetic energy source to thesecond voltage potential.

In accordance with a further embodiment of the present invention, amethod of acquiring imaging data at more than one chromatic energycomprises the step of projecting a first beam of electromagnetic energyalong a projection path toward a subject. The method further includesthe step of positioning a first filter in the projection path duringprojection of the first beam. The method also includes projecting asecond beam of electromagnetic energy along the projection path towardthe subject and includes the step of positioning a second filter in theprojection path during projection of the second beam of electromagneticenergy.

In accordance with yet a further embodiment of the present invention, acomputer readable storage medium has a computer program stored thereon.The computer program represents a set of instructions that when executedby a computer causes the computer to energize high frequencyelectromagnetic energy source to a first voltage to cause the highfrequency electromagnetic energy source to project a first beam ofelectromagnetic energy toward the subject to be scanned. The computer isfurther caused to position a first filter between the high frequencyelectromagnetic energy source and the subject during energization of thehigh frequency electromagnetic energy source to the first voltage. Theset of instructions further causes the computer to energize the highfrequency electromagnetic energy source to a second voltage to cause thehigh frequency electromagnetic energy source to project a second beam ofelectromagnetic energy toward the subject and position a second filterbetween the electromagnetic energy source and the subject duringenergization of the high frequency electromagnetic energy source to thesecond voltage.

In accordance with yet another embodiment of the present invention, afiltering apparatus for a radiation emitting imaging system is provided.The filtering apparatus includes a hub having a number of connectionports and a first filter connected to the hub at a first connection portand a second filter connected to the hub at a second connection port.The first filter is configured to have a first filtering power and thesecond filter is configured to have a second filtering power.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A CT system comprising: a rotatable gantry having an opening forreceiving a subject to be scanned; an HF electromagnetic energy sourceconfigured to project a number of HF electromagnetic energy beams towardthe subject; a generator configured to energize the HF electromagneticenergy source to at least a first energy state and a second energystate; a first filtering apparatus comprising a number of HFelectromagnetic energy filters positional between the HF electromagneticenergy source and the subject, the number of HF electromagnetic energyfilters including at least a first filter and a second filter whereinthe first filter is positioned between the HF electromagnetic energysource and the subject by rotation of the first filtering apparatusbefore the HF electromagnetic energy source is energized to the firstenergy state and the second filter is positioned between the HFelectromagnetic energy source and the subject by rotation of the firstfiltering apparatus before the HF electromagnetic energy source isenergized to the second energy state; and wherein a plane of the firstfilter is parallel with an axis of rotation of the first filteringapparatus, and a plane of the second filter is parallel with the axis ofrotation of the axis of rotation of the first filtering apparatus. 2.The CT system of claim 1 wherein the plane of the first filter ispositioned orthogonal to the HF electromagnetic energy source when theHF electromagnetic energy source is energized to the first energy state,and the plane of the second filter is positioned orthogonal to the HFelectromagnetic energy source when the energy source is energized to thesecond energy state.
 3. The CT system of claim 1 wherein the HFelectromagnetic energy source and the number of HF electromagneticenergy filters are rotatable about the subject.
 4. The CT system ofclaim 1 further comprising: a set of HF electromagnetic energy detectorsconfigured to generate a set of electrical signals indicative of HFelectromagnetic energy attenuated by the subject; a DAS configured toreceive the set of electrical signals; and an image reconstructorconnected to the DAS and configured to reconstruct an image of thesubject from the electrical signals received by the DAS.
 5. The CTsystem of claim 1 incorporated into a medical imaging device and whereinthe subject is a medical patient.
 6. The CT system of claim 1 whereinthe movable table is configured to convey articles through the openingwherein the articles include pieces of luggage/baggage and packages. 7.The CT system of claim 6 incorporated into at least one of an airportinspection apparatus and a postal inspection apparatus.
 8. The CT systemof claim 1 further comprising a second filtering apparatus having a backsurface, a first top surface, and a second top surface, wherein the twotop surfaces are substantially co-planar and in a stepped relationshipwith one another, wherein a first filtration thickness is formed betweenthe first top surface and the back surface, and a second filtrationthickness is formed between the second top surface and the back surface,and wherein one of the first filtration thickness and the secondfiltration thickness is positioned between the HF electromagnetic energysource and the subject before the HF electromagnetic energy source isenergized to either one of the first energy state and the second energystate.
 9. The CT system of claim 1 wherein the first energy state issubstantially equal to the second energy state.
 10. A method ofacquiring imaging data at more than one chromatic energy comprising thesteps of: projecting a first beam of electromagnetic energy along asingle projection path toward a subject to be scanned; positioning afirst filter of a first filtering apparatus in the single projectionpath before projection of the first beam; projecting a second beam ofelectromagnetic energy along the single projection path toward thesubject; and positioning a second filter of the first filteringapparatus in the single projection path before projection of the secondbeam, by rotation of the first filtering apparatus about an axis ofrotation, wherein a plane of the first filter is parallel with the axisof rotation, and a plane of the second filter is parallel with the axisof rotation.
 11. The method of claim 10 further comprising the steps of:energizing an HF electromagnetic energy source to a first voltage togenerate the first beam of electromagnetic energy; rotating the firstfiltering apparatus about the axis of rotation to position the firstfilter along a path of rotation such that the first filter is in theprojection path before energization of the HF electromagnetic energysource to the first voltage; energizing the HF electromagnetic source toa second voltage to generate the second beam of electromagnetic energy;and rotating the first filtering apparatus about the axis of rotation toposition the second filter along the path of rotation such that thesecond filter is in the projection path before energization of the HFelectromagnetic energy source to the second voltage.
 12. The method ofclaim 10 wherein the step of rotating the first filtering apparatusfurther comprises positioning the plane of the first filter orthogonalto the single projection path during projection of the first beam ofelectromagnetic energy before the energy source is energized to thefirst voltage, and positioning the plane of the second filter orthogonalto the single projection path before projection of the second beam ofelectromagnetic energy when the energy source is energized to the secondvoltage.
 13. The method of claim 10 further comprising the step ofacquiring imaging data with the first beam of electromagnetic energybeing substantially equal to the second beam of electromagnetic energy.14. The method of claim 10 further comprising positioning a secondfiltering apparatus, the second filtering apparatus having a backsurface, a first top surface, and a second top surface, wherein the twotop surfaces are substantially co-planar and in a stepped relationshipwith one another, wherein a first filtration thickness is formed betweenthe first top surface and the back surface, and a second filtrationthickness is formed between the second top surface and the back surface,and wherein one of the first filtration thickness and the secondfiltration thickness is positioned between the HF electromagnetic energysource and the subject before projecting one of the first beam ofelectromagnetic energy and the second beam of electromagnetic energy.15. A computer readable storage medium having a computer program storedthereon and representing a set of instructions that when executed by acomputer causes the computer to: energize an HF electromagnetic energysource to a first voltage to cause the HF electromagnetic energy sourceto project a first beam of electromagnetic energy toward a subject to bescanned; position at least a first filter of a first of a firstfiltering apparatus, between the HF electromagnetic energy source andthe subject before energization of the HF electromagnetic energy sourceto the first voltage, wherein a plane of the first filter is parallelwith an axis of rotation of the first filtering apparatus; energize theHF electromagnetic energy source to a second voltage to cause the HFelectromagnetic energy source to project a second beam ofelectromagnetic energy toward the subject; and rotate the firstfiltering apparatus about the axis of rotation and position a secondfilter of the first filtering apparatus between the HF electromagneticenergy source and the subject before energization of the HFelectromagnetic energy source to the second voltage, wherein a plane ofthe second filter is parallel with the axis of rotation of the firstfiltering apparatus.
 16. The computer readable storage medium of claim15 wherein the set of instructions further causes the computer to rotatethe first filter and the second filter about the subject along a commonpath of rotation.
 17. The computer readable storage medium of claim 15wherein the set of instructions further causes the computer to rotatethe first filter about the subject along a first path of rotation androtate the second filter about the subject along a second path ofrotation.
 18. The computer readable storage medium of claim 15incorporated into a medical imaging apparatus configured to acquirediagnostic imaging data of a medical patient.
 19. The computer readablestorage medium of claim 15 incorporated into a non-invasive parcelinspection apparatus including at least one of a postal inspectionapparatus and a baggage inspection apparatus.
 20. The computer readablestorage medium of claim 15 wherein the set of instructions furthercauses positioning a second filtering apparatus, the second filteringapparatus having a back surface, a first top surface, and a second topsurface, wherein the two top surfaces are substantially co-planar and ina stepped relationship with one another, wherein a first filtrationthickness is formed between the first top surface and the back surface,and a second filtration thickness is formed between the second topsurface and the back surface, and wherein one of the first filtrationthickness and the second filtration thickness is positioned between theHF electromagnetic energy source and the subject before the HFelectromagnetic energy source is energized to either one of the firstvoltage or the second voltage.
 21. A filtering apparatus for a radiationemitting imaging system, the filtering apparatus comprising: an axis ofrotation; a first filter connected to the filtering apparatus, the firstfilter having a first filtering power; and a second filter connected tothe filtering apparatus, the second filter having a second filteringpower; and wherein a plane of the first filter is parallel with the axisof rotation, and a plane of the second filter is parallel with the axisof rotation.
 22. The filtering apparatus of claim 21 wherein a hub isconfigured to rotate the first filter about the axis of rotation into apath of HF electromagnetic energy before an HF electromagnetic energysource is energized to a first voltage, and rotate the second filterabout the axis of rotation into the path of HF energy before the HFelectromagnetic energy projection source is energized to a secondvoltage.
 23. The filtering apparatus of claim 21 wherein the plane ofthe first filter is positioned orthogonal to the HF electromagneticenergy source when the energy source is energized to the first voltage,and the plane of the second filter is positioned orthogonal to the HFelectromagnetic energy source when the energy source is energized to thesecond voltage.
 24. The filtering apparatus of claim 21 wherein the hubis cylindrical or spherical, the filtering apparatus further comprisinga third filter connected to the hub and a fourth filter connected to thehub, the first, the second, the third and the fourth filters havingdiffering filtering powers and the third connection port beingpositioned 90° along the hub from the fourth connection port; whereinthe first, the second, the third, and the fourth filters are snap-fit,bolted, or integrated as a single integral body as attached to the hub.25. The filtering apparatus of claim 21 further comprising an additionalfiltering apparatus, the additional filtering apparatus having a backsurface, a first top surface, and a second top surface, wherein the twotop surfaces are substantially co-planar and in a stepped relationshipwith one another, wherein a first filtration thickness is formed betweenthe first top surface and the back surface, and a second filtrationthickness is formed between the second top surface and the back surface,and wherein one of the first filtration thickness and the secondfiltration thickness is positioned between the HF electromagnetic energysource and the subject before the HF electromagnetic energy source isenergized to either one of a first voltage or a second voltage.